Vlll MORPHOLOGY OF SHELL. 



Sections. Shell globose, Shell depressed, 



" Dentellari't " nuxdenticulata, deniiem, lychnuchus. 



Thelidomus emarginata, petltiana, lima. 



Pleurodonte bronni, anomala, ' pvracutissima. 



Stenotrema stenotrema, monodon, spinosa. 



Axina montfortiana, magister, siquijorensis, 



That characters of contour are valueless for distinguishing gen- 

 era in Helices is now conceded by students of the living groups, 

 but palaeontologists still use them ; and for this reason the above 

 tables are given. 



The sculpture of Helices, like the contour, affords valuable spe- 

 cific characters, being subject to a wide range of mutation. Shells 

 may be either smooth, obliquely striate, ribbed, decussated, granu- 

 lated, malleated or hairy ; and frequently several varieties of sculp- 

 ture characterize different species of one genus, thus : 



Genus. granulate, spirally striate, 



Helicigona lapicida, arbustorum, 



Polygyra palliata, albolabris, 



Epiphragmophora tudiculata, inter cisa, 



Pleurodonte lima, petitiana, 



ribbed, hairy, smooth. 



gobanzi, setosa, cingulata. 



obstricta, hirsuta, jejuna, 



eircumcarinata, remondi, mormonum. 



scabrosa, auridens, marginella. 



Sometimes upon a smooth or granulate surface there are papillae 

 or hairs arranged in regular obliquely decussating series, or in 

 quincunx. This occurs in some species of Chloritis, Helicigona, 

 Thysanophora, Lysinoe, Hygromia, etc. Some genera exhibit a 

 wide range of variation in texture and color, but in most cases this 

 is correllated with the habits of the species. Tree living snails are, 

 as a rule, bright colored and tend to become elevated or conical, 

 while ground snails are duller or brown, and usually depressed* 

 Some genera, like Helieostyla in the Philippines and Cepolis in the 

 West Indies, contain both arboreal and terrestrial forms, and con- 

 sequently appear, on superficial observation, to be composed of very 

 incongruous elements. 



The embryonic shell (the portion formed within Ihe egg), is found 

 to vary greatly in size, and its extent compared to that of the adult 



